Why do I get no response from the Alnor HRU on the Modbus bus?

The Alnor HRU requires the HRQ-Modbus gateway from Alnor to bridge its wireless control system to Modbus RTU. Without this gateway, the unit does not appear on the RS-485 bus. Also verify that the baud rate is set to 19200 and parity is set to odd (8E1).

Temperature readings show incorrect or very large values. What is wrong?

Temperature registers use float32 format spanning two consecutive Modbus registers. Ensure the word order (big endian or little endian) in your ModbusCloud gateway configuration matches the byte order expected by the Alnor HRQ-Modbus gateway.

Can I use this template with any Alnor HRU?

This template works only with Alnor HRU units equipped with the wireless control system. Older units without this system are not supported. Check your HRU model against the supported devices list in the template.

How do I reset the filter timer after replacing the filter?

The template includes a filter timer reset register. Write the reset command from the ModbusCloud dashboard or automation rule after physically replacing the filter in the unit.

Free Modbus template: Alnor HRU

This template provides full Modbus RTU monitoring and control of Alnor HRU heat recovery ventilation units. It reads temperatures, airflow, humidity, CO2, fan speeds, and filter status. It also allows setting ventilation profiles, bypass mode, and fireplace demand.

Name	Address	Function	Type	Unit	Access
Actual Ventilation Speed 0 = OFF, 1 = Speed 1(low), 2 = Speed 2(medium), 3 = Speed 3(high), 23 = Boost speed, 24 = Auto mode	41500	FC03	uint16		R/W
Error code 0 - No fault, 1 - Nonspecific fault, 2 - Emergency temperature stop, 3 - Fan 1 supply error, 4 - X22 sensor error, 5 - X23 sensor error, 6 - X21 sensor error, 7 - X20 sensor error, 8 - Fan 2 error, 254 - Binding mode active, 255 - Device identification active_	41003	FC03	uint16		R
Exhaust fan speed	41001	FC03	uint16	%	R
Supply fan speed	41002	FC03	uint16	%	R
Extract Temperature	41005	FC03	float32	°C	R
Inlet Temperature	41007	FC03	float32	°C	R
Exhaust Temperature	41009	FC03	float32	°C	R
Supply Temperature	41011	FC03	float32	°C	R
Preheater status 0 = OFF, 1 = ON	41013	FC03	uint16		R
Defrost 0 - defrost inactive, 1 - defrost active	41015	FC03	uint16		R
Bypass position 0_ = Fully closed, 100_ = Fully open	41051	FC03	uint16	%	R
Humidity indoor	41017	FC03	uint16	%	R
Filter status 0 = Filter OK, 1 = Filter_	41014	FC03	uint16		R
Supply air flow	41019	FC03	float32	m³/h	R
Exhaust air flow :	41021	FC03	float32	m³/h	R
CO2 indoor Based on readings from Alnor external CO2 sensor	41025	FC03	uint16	ppm	R
Filter Time Remaining	41040	FC03	uint16	days	R
Bypass Mode 0 = manual closed (0), 100 = manual open (100), FFh= Auto	41050	FC03	uint16	%	R
Supply fan speed - Standby Min: 0, Max: 40	42001	FC03	uint16	%	R/W
Exhaust fan speed - Standby Min: 0, Max: 40	42002	FC03	uint16	%	R/W
Supply fan speed - Speed 1 Min: 0, Max: 80	42003	FC03	uint16	%	R/W
Exhaust fan speed - Speed 1 Min: 0, Max: 80	42004	FC03	uint16	%	R/W
Supply fan speed - Speed 2 Min: 0, Max: 100	42005	FC03	uint16	%	R/W
Exhaust fan speed - Speed 2 Min: 10, Max: 100	42006	FC03	uint16	%	R/W
Supply fan speed - Speed 3 Min: 0, Max: 100	42007	FC03	uint16	%	R/W
Exhaust fan speed - Speed 3 Min: 10, Max: 100	42008	FC03	uint16	%	R/W
Fireplace demand state 0 = inactive, 1 = active	41551	FC03	uint16		R/W
Reset Filter Time 0 = reset	42000	FC06	uint16		W
Bypass mode 0 = closed, 100 = open, FFh = AUTO	41550	FC06	uint16	%	W

Alnor HRU

Overview

Key readings

Available commands

Use cases

Installation

Things to watch for

Registers (29)

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